This invention relates generally to semiconductor laser arrays and, more particularly, to coherent diffraction-coupled laser arrays. Linear arrays of semiconductor arrays have been used for some years as a technique for increasing the power output from semiconductor lasers. For a better understanding of the invention, some background information on semiconductor lasers and laser arrays is first presented.
Basically, a semiconductor laser is a multilayered structure composed of different types of semiconductor materials, chemically doped with impurities to give them either an excess of electrons (n type) or an excess of electron vacancies or holes (p type). The basic structure of the semiconductor laser is that of a diode, having an n type layer, a p type layer, and an undoped active layer sandwiched between them. When the diode is forward-biased in normal operation, electrons and holes combine in the region of the active layer, and light is emitted. The layers on each side of the active layer have a lower index of refraction than the active layer, and function as cladding layers to confine the light in the plane of the active layer. Various techniques are used to confine the light in a lateral direction as well, and crystal facets are located at opposite ends of the structure, to provide for repeated reflections of the light back and forth in a longitudinal direction. If the diode current is above a threshold value, lasing takes place and light is emitted from one or both of the facets, in the plane of the active layer.
The power of a single semiconductor laser is limited by various physical parameters, but a linear array of coherent emitters, coupled to a corresponding array of evanescently coupled optical waveguides, has been employed to increase the total output power. Instead of the evanescently coupled waveguides, an array might consist of otherwise laterally confined channels and gain regions. One significant disadvantage of linear laser arrays is that the far-field radiation distribution pattern is typically double-lobed.
In general, a coherent array of coupled laser channels can oscillate in one or more of multiple possible configurations, known as array modes. In the most desirable array mode, all of the emitters oscillate in phase. This is known as the 0.degree.-phase-shift array mode, or the in-phase supermode, and it produces a far-field pattern in which most of the energy is concentrated in a single narrow lobe whose angular width is limited, ideally, only by the diffraction of light. A less desirable far-field distribution is obtained when adjacent laser emitters are 180.degree. out of phase. This is the 180.degree.-phase-shift array mode, or the out-of-phase supermode, and it produces two relatively widely spaced lobes in the far-field distribution pattern. Multiple additional modes exist between these two extremes, depending on the phase alignment of the various emitters. Most laser arrays operate in two or three array modes simultaneously and produce one or more beams that are typically two or three times wider than the diffraction limit, with a correspondingly lower peak intensity.
There is an inherent tendency for a linear laser array to oscillate in the out-of-phase supermode. In a conventional linear array, both the optical energy and material gain are concentrated in the same channels, and alternate periodically with lossy interchannel regions. Any array mode having a nonzero intensity in the lossy regions between laser elements will therefore have a higher modal loss, compared with a mode which does not have a nonzero intensity in the regions between laser elements. It is only in the out-of-phase supermode that the field diminishes to zero in all of the regions between lasing elements. Therefore, the interchannel loss contribution to the round-trip propagation loss in the cavity for oscillation in this mode is lower than for oscillation in the other modes. It is apparently for this reason, often referred to in the literature as interchannel evanescent coupling, that the out-of-phase supermode emerges as the dominant mode in most coherent laser arrays. Although this mode minimizes propagation losses, it produces a less than desirable far-field pattern, and much research effort has been directed to structuring the laser array to favor the in-phase supermode and to discriminate against the out-of-phase supermode.
Various diode array geometries have been proposed to force oscillation in a more desirable in-phase supermode. Some of these approaches employ chirped arrays, i.e. arrays in which non-uniform contact stripe width, non-uniform current drives, or non-uniform center-to-center spacings, provide a better spatial match between the in-phase mode and the gain distribution acorss the array width. Other approaches may involve phase interference effects. Specifically, the approach with which this invention is concerned employs the diffraction-coupled array. In a diffraction coupled array, the outputs of an array of parallel waveguides interact through beam diffraction in a laterally unguided region. Light from each waveguide is reflected from an end facet of the unguided region and injected back into the waveguide and into its neighboring waveguides, prinicipally the immediately adjacent waveguides.
If the geometry of the unguided region is correctly proportioned, light will be injected back into the neighboring waveguides with a phase angle of approximately 2.pi. or an integral multiple of 2.pi.. The in-phase supermode of operation is then reinforced and the array will operate in the in-phase mode and produce a more desirable single-lobed far-field pattern.
This invention is concerned with a specific application of laser arrays in which laser light output must be pulsed for purposes of modulation, as in pulse-code-modulation communication systems. In the past, this has been accomplished by electrically pulsing the current injected into the laser, but this approach has the disadvantage that stable operation of the lasers must be reestablished for each pulse. Lasers are typically subject to transient modes of operation when first turned on, and before stability is achieved. For this reason, the modulation is limited to several hundreds of megahertz. To increase the modulation frequency, it is desirable to run the lasers continuously if possible, and some pulse modulation techniques employ optical or electrooptical elements external to the laser array, to achieve switching of the laser output. Prior to this invention, however, there has been no satisfactory technique for rapidly pulse-modulating a semiconductor laser array directly. The present invention is directed to this end.